The activities of the microsomal mixed-function mono-oxygenases detoxifying and activating carcinogenic polycyclic hydrocarbons are key factors in the initiation of chemical tumorigenesis. The constitutive level and the inducibility of these enzymes are under tight genetic control, but are also subject to modification by exogenous chemicals, $ i.e., inducers and inhibitors. It is the aim of this project to study the regulatory mechanism of enzyme activity in cells in culture, to dissect the genetic components of regulation in somatic cell hybrids and to explore the mechanism of induction by exogenous compounds.